1. Field of the Invention
The present invention is generally related to semi-solid die casting and, more particularly, to a process for reducing the likelihood that impurities and air entrapment will adversely affect the quality of a die cast component.
2. Description of the Related Art
Those skilled in the art of die casting and, more particularly, die casting of semi-solid material are familiar with various processes and procedures associated with die casting methods.
U.S. Pat. No. 3,650,312, which issued to Allen on Mar. 21, 1972, describes a hybrid casting-hot working process for shaping magnesium, aluminum, zinc and other die casting metals. Molten metal is thickened by addition of high surface area aerogel powder to produce a thixotropic mass which is workable at low working forces and retains its worked shape after removal of the applied working forces. Cooling to below melting temperatures solidifies the mass into a product having the metallurgical structural characteristics of a forged metal notwithstanding the use of casting methods in fabrication.
U.S. Pat. No. 5,879,478, which issued to Loue et al. on Mar. 9, 1999, describes a process for semi-solid forming of thixotropic aluminum-silicon-copper alloys. It relates to an aluminum alloy for thixoforming with a particular composition which, when reheated to the semi-solid state to the point at which a liquid fraction ratio between 35 and 55% is obtained, has an absence of non-remelted polyhedral silicon crystals.
U.S. Pat. No. 5,219,018, which issued to Meyer on Jun. 15, 1993, describes a method of producing thixotropic metallic products by continuous casting with polyphase current electromagnetic agitation. The invention relates to a continuous casting method for producing thixotropic metallic alloys containing degenerated dendrites. It consists of casting the liquid metal in a movable occluded mould consisting of a hot upstream zone produced from insulating material and a cooled downstream zone in which the metal solidifies, while carrying out by means of a sliding magnetic field, obtained by a series of polyphase inductors, an electromagnetic agitation which causes the dendrites formed in the cold zone to pass into the hot zones where they change to nodules by superficial refusion.
U.S. Pat. No. 5,865,238, which issued to Carden et al. on Feb. 2, 1999, describes a process for die casting of metal matrix composite materials from a self-supporting billet. The billet is formed of an aluminum alloy matrix and ceramic particles. It is heated in an oxygen containing atmosphere forming an aluminum oxide surface and softening the matrix alloy. The semi-solid billet then is compressed in a die casting sleeve and the softened matrix material is displaced into a die to form the shape.
U.S. Pat. No. 6,098,700, which issued to Carden et al. on Aug. 8, 2000, describes an apparatus for die casting of metal matrix composite materials from a self-supporting billet. The billet is composed essentially of a metal alloy matrix and dispersed ceramic particles. It comprises heating means to soften the metal alloy, a horizontal plunger to drive and to compress the billet, a die through which the softened metal matrix and ceramic particles are formed into a shape defined by the interior surface of the die, and cooling means to maintain the temperature of the interior surface of the die at a predetermined temperature.
U.S. Pat. No. 6,399,017, which issued to Norville et al. on Jun. 4, 2002, discloses a method and apparatus for containing and ejecting a thixotropic metal slurry. A container system includes a vessel for holding a thixotropic semi-solid aluminum alloy slurry during its processing as a billet and an ejection system for cleanly discharging the processed thixotropic semi-solid aluminum billet. The crucible is preferably formed from a chemically and thermally stable material. The crucible defines a mixing volume. The crucible ejection mechanism may include a movable bottom portion mounted on a piston or may include a solenoid coil for inducing an electromotive force in the electrically conducting billet for urging it from the crucible.
U.S. Pat. No. 6,402,367, which issued to Lu et al. on Jun. 11, 2002, discloses a method and apparatus for magnetically stirring a thixotropic metal slurry. The aluminum alloy comprises a first solid particulate phase suspended in a second liquid phase so as to maintain its thixotropic character by degenerating forming dendritic particles into spheroidal particles while simultaneously equilibrating the melt temperature by quickly transferring heat between the melt and its surroundings. The melt is stirred by a magnetomotive force field generated by a stacked stator assembly.
U.S. Pat. No. 6,432,160, which issued to Norville et al. on Aug. 13, 2002, discloses a method and apparatus for making a thixotropic metal slurry. It comprises simultaneously controlledly cooling and stirring the melt to form solid particles of a first phase suspended in a residual liquid second phase. Vigorous stirring of the metallic melt results in the formation of degenerate dendritic particles having substantially spheroidal shapes.
U.S. Pat. No. 6,637,927, which issued to Lu et al. on Oct. 28, 2003, discloses a method and apparatus for magnetically stirring a thixotropic metal slurry. A melt is stirred by a magnetomotive force field generated by a stacked stator assembly. The stacked stator assembly includes a stator ring adapted to generate a linear/longitudinal magnetic field positioned between two stator rings adapted to generate a rotational magnetic field. The stacked stator rings generate a substantially spiral magnetomotive mixing force and define a substantially cylindrical mixing region therein.
U.S. Pat. No. 6,932,938, which issued to Norville et al. on Aug. 23, 2005, discloses a method and apparatus for containing and ejecting a thixotropic metal slurry. A crucible is preferably formed from a chemically and thermally stable material such as graphite or a ceramic. The crucible defines a mixing volume. The crucible ejection mechanism may include a movable bottom portion mounted on a piston or may include a solenoid coil for inducing an electromotive force in the electrically conducting billet for urging it from the crucible.
U.S. Pat. No. 6,962,189, which issued to Buckley on Nov. 8, 2005, describes a method of making precision castings using thixotropic materials. Precision castings requiring a fine finish and having complex internal geometries can be produced by casting a semi-solid thixotropic metal alloy within or about a meltaway material component in the form of a core and/or a die insert that has a lower melting point than the solid to semi-solid transition temperature of the thixotropic alloy.
U.S. Pat. No. 7,132,077, which issued to Norville et al. on Nov. 7, 2006, discloses a method and apparatus for containing and ejecting a thixotropic metal slurry. During processing, a molten aluminum alloy precursor is transferred into the crucible and vigorously stirred and controllably cooled to form a thixotropic semi-solid billet. Once the billet is formed, the ejection mechanism is activated to discharge the billet from the crucible. The billet is discharged into a shot sleeve and immediately placed in a mold and molded into a desired form.
U.S. Pat. No. 7,296,611, which issued to Hirai et al. on Nov. 20, 2007, describes a method and apparatus for manufacturing metallic parts by die casting. An injection molding apparatus includes a melt furnace and a metal supply system located in the melt furnace. The metal supply system includes a pump. The injection molding apparatus also includes a first metal inlet from the melt furnace to the metal supply and a vertical injection mechanism adapted to inject liquid metal into a die system. The injection molding apparatus also includes a second metal inlet from the metal supply system to the vertical injection mechanism.
The patents described above are hereby expressly incorporated by reference in the description of the present invention.